In metro and short-reach optical networks, such as inter- and intra-data centers, low power consumption, low cost, and high density are three important factors for optoelectronics components. However, optical networks commonly employ a large number of optical transceivers, which are the most expensive components and consume the most power. Optical transceivers may include components, such as high-speed analog-to-digital converters (ADCs), high-speed digital-to-analog converters (DACs), optical modulators, and radio frequency (RF) drivers. In order to meet the higher and higher bandwidth requirements due to the exponential growth of Internet traffic, advanced modulation formats that are used in optical long haul transmission systems, such as direct detection-differential quadrature phase-shift keying (DD-DQPSK), dual-polarization quadrature phase-shift keying (DP-QPSK), and 16 quadrature amplitude modulation (16QAM), are also deployed in short-reach networks. However, most of the optical components used in long haul transmission systems may not satisfy the low power consumption, low cost, and high density requirements in short-reach and metro optical networks.
Most of the advanced modulation formats in commercial optical equipment are based on Mach-Zehnder modulators (MZMs). The working principle of MZMs is to modulate the optical phase difference between two waveguides, which then interfere constructively or destructively to achieve amplitude modulation and phase modulation on the output. For example, MZMs are employed to generate highly stable optical signals as described in H. Kiuchi, et. al., “High Extinction Ratio Mach-Zehnder Modulator Applied to a Highly Stable Optical Signal Generator,” Institute of Electronics and Electrical Engineers (IEEE) Transactions of Microwave Theory and Techniques, Vol. 55, No. 9, September 2007, pp. 1694-1972, which is incorporated by reference. MZMs that are commonly employed in industry may include lithium nobiate (LiNbO3)-based MZMs, indium phosphide (InP)-based MZMs, and silicon (Si)-based MZMs. Due to the high-density requirement in optical transmitters, LiNbO3-based transmitters may not be suitable for short-reach applications. The highly integrated Si-based transmitters and InP-based transmitters are more suitable for short-reach applications.
FIG. 1 is a schematic diagram of a conventional optical in-phase quadrature-phase quadrature phase-shift keying (IQ QPSK) modulator 100 which modulates an input signal 20 emitted by laser diode (LD) 10 according to a digital signal to produce a modulated output signal 30. Modulator 100 comprises three MZMs 110, 120, and 130. MZMs 120 and 130 are referred to as child modulators and MZM 110 is referred to as a parent modulator. MZMs 120 and 130 are positioned in parallel with each other. MZM 120 is configured to generate in-phase (I) components according to RF driver 121. MZM 130 is configured to generate quadrature-phase (Q) components according to RF driver 131. The output of MZM 130 passes through phase shifter 133, and output signals 122 and 132 are combined, resulting in modulated output data signal 30. MZMs 120 and 130 operate at null points, which are transmission minimum points, and MZM 110 operates at a quadrature point, which is a 3 decibel (dB) loss point. FIGS. 2A-2C illustrate the output signals, in the form of constellation diagrams, as generated by the MZMs of IQ QPSK modulator 100. FIG. 2A illustrates a constellation diagram of output signal 122 of MZM 120. FIG. 2B illustrates a constellation diagram of output signal 132 of MZM 130. FIG. 2C illustrates a constellation diagram of an output signal 30 of modulator 100.
Modulator 100 may be employed to generate 16QAM and higher order modulation signals by configuring RF drivers 121 and 131 to generate multi-level outputs as described in T. Sakamoto, et. al., “50-Gb/s 16 QAM by a Quad-Parallel Mach-Zehnder Modulator,” Optical Communication—Post Deadline Papers, 33rd European Conference, 2007, pp. 1-2, which is incorporated by reference. Modulator 100 may also be employed to generate 16QAM and higher order modulation signals by cascading modulator 100 and an optical phase modulator (PM) as described in M. Serbay, et. al., “Implementation of Differential Precoder for High-Speed Optical DQPSK Transmission,” Electronics Letters, volume 40, issue 20, 30 Sep. 2004, pp. 1288-1289 (Serbay), which is incorporated by reference.